1. Field of the Invention
This invention relates to an aluminum bearing alloy generally used in bearings for high-power engines of automobiles and industrial machines.
2. Description of Related Art
An aluminum bearing alloy of the above-described type usually contains Sn giving conformability thereto. Pb also gives conformability to the aluminum bearing alloy. However, Pb cannot easily be distributed uniformly in the alloy and is one of injurious metals. Pb is not used for these reasons.
A plate-shaped cast aluminum bearing alloy containing Sn is cladded to a back steel plate to thereby be used for bearing. In manufacturing bearings using the aforesaid aluminum bearing alloy, annealing is indispensable after cladding to improve the toughness of the bearing alloy and the adhesive strength between the bearing alloy and the back steel plate. When annealing places the aluminum bearing alloy containing Sn under the condition of a high temperature, Al grains and Sn phases in the alloy structure are coarsened such that a high-temperature hardness and a fatigue strength of the aluminum bearing alloy are reduced.
In view of the aforesaid problem, it has been suggested that minute hard particles, for example, Si particles having a diameter less than 5 xcexcm should be contained in the aluminum alloy so that Sn phases and Al grains are prevented from being coarsened and so that an Al matrix is strengthened, whereupon the aluminum bearing alloy provides a high bearing performance under the conditions of high load and high temperature.
Furthermore, each of JP-A-58-64332 and JP-A-58-67841 proposed containment of hard particles from a point of view different from strengthening the Al matrix for the same purpose as described above. The former, JP-A-58-64332, discloses that Si particles are used as hard particles and that the size and distribution of the Si particles are controlled so that the bearing characteristics, particularly conformability and anti-seizure property are rapidly improved under the conditions of high load and high temperature. Conventional Si particles mainly have a diameter of less than 5 xcexcm. However, coarsened Si particles having a diameter ranging between 5 and 40 xcexcm can be obtained as the result of improvements in the conditions of the thermal treatment etc. The coarsened Si particles scrape off protrusions on the surface of the shaft and an edge such as burrs around nodular graphite on the surface of the shaft at an initial stage of sliding, resulting in smooth sliding of the bearing (lapping). Consequently, seizure is prevented even under the condition of high load, whereupon the anti-seizure property can rapidly be improved.
On the other hand, the latter publication, JP-A-58-67841, discloses that particles consisting of or including at least one of Mn, Fe, Mo, Ni, Zr, Co, Ti, Sb, Cr, and Nb are crystallized as a separate metal or deposited or yielded as an intermetallic compound containing Mn or the like with Al. It is further disclosed that the crystallized separate metal or deposited intermetallic compound having a diameter ranging between 5 and 40 xcexcm has the same effect as the Si particles in the above-described JP-A-58-46332. For example, when added to an Al alloy, Mn or the like is crystallized as a separate metal, or hard particles containing Mn or the like certainly exist although no crystalline can be specified. Thus, JP-A-58-46332 suggests Mn, Fe, etc. as an element accelerating generation of hard particles in an alloy excluding Si.
Regarding an alloy containing Si, only Si particles are conventionally hard particles crystallized in the aluminum bearing alloy. Regarding an alloy excluding Si, an added metal is crystallized in its original condition or crystallization of a binary intermetallic compound is suggested without crystallized substance being specified. Additionally, coarsened hard particles cause the lapping of the shaft, whereupon anti-seizure property is improved.
Hard particles contained in Al are generally distributed uniformly for the strengthening purpose. A large effect is achieved as the size of the particles becomes smaller. Accordingly, when the hard particles such as Si are coarsened as in the aforesaid publications, the strength of the Al matrix is reduced such that the fatigue resistance is reduced. In other words, regarding the size of crystallized particles, when the diameter of the crystallized particles is reduced in order that the fatigue resistance may be improved, the anti-seizure property cannot be improved. On the contrary, when the diameter of the crystallized particles is increased in order that the anti-seizure property may be improved, the fatigue resistance cannot be improved.
Therefore, an object of the present invention is to provide an aluminum bearing alloy in which the anti-seizure property thereof can be improved by hard particles without reduction in the fatigue resistance thereof.
The above-described object can be achieved by crystallizing a ternary-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe or a multi-element intermetallic compound with the base of Alxe2x80x94Sixe2x80x94Fe in an aluminum bearing alloy. More specifically, the present invention provides an aluminum bearing alloy comprising, by mass, 3 to 40% Sn, 0.5 to 7% Si, 0.05 to 2% Fe, balance of Al, and unavoidable impurities, wherein a ternary-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe and Si particles are contained as hard particles.
In a first preferred form, the aluminum bearing alloy further comprises at least one or more of Mn, V, Mo, Cr, Co, Ni and W in an amount or a total amount of 0.01 to 3%. In the alloy, said one or more of Mn, V, Mo, Cr, Co, Ni and W are added to the Alxe2x80x94Sixe2x80x94Fe such that a multi-element intermetallic compound is contained as hard particles.
In a second preferred form, the hard particles have a maximum diameter ranging between 1 and 20 xcexcm on a bearing surface and a number of the hard particles for every 1 mm2 ranges between 6 and 100.
In a third preferred form, the alloy contains by mass, 0.01 to 2% of at least one or more of B, Ti and Zr.
In a fourth preferred form, the alloy contains by mass, 0.1 to 5% of at least one or more of Cu, Mg and Zn.
Crystallization of the intermetallic compound of Alxe2x80x94Sixe2x80x94Fe will now be described. Firstly, in a well-konwn ternary-element state of Alxe2x80x94Sixe2x80x94Fe, a ternary-element eutectic compound is in the phase of Fe2Al9Si2 and has a crystallizing temperature of 573xc2x0 C. A eutectic reaction is shown as Lxe2x86x92Sn+Si+Fe2Al9Si2.
FIG. 1 shows the structure of an alloy of Alxe2x80x94Snxe2x80x94Sixe2x80x94Fe in the invention claimed in claim 1. As obvious from FIG. 1, a ternary-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe is crystallized as a eutectic compound in the shape of a needle, bar or plate. FIG. 2 shows the structure of an alloy in the invention claimed in claim 2, for example, an alloy of Alxe2x80x94Snxe2x80x94Sixe2x80x94Fexe2x80x94Mn. A multi-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fexe2x80x94Mn is seen in FIG. 2. FIG. 3 shows the structure of an alloy of Alxe2x80x94Snxe2x80x94Sixe2x80x94Fexe2x80x94Cr. A multi-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fexe2x80x94Cr is seen in FIG. 3.
A multi-element intermetallic compound such as Alxe2x80x94Sixe2x80x94Fexe2x80x94Mn or Alxe2x80x94Sixe2x80x94Fexe2x80x94Cr has various forms and is crystallized as a eutectic compound in which compounds including those having shapes of a needle, bar and plate each with three-dimensionally unspecified thickness are combined together in a complicated form. The ternary-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fe and the multi-element intermetallic compound containing Alxe2x80x94Sixe2x80x94Fe as a base are exceedingly stable, and its basic shape is not changed even by the heat treatment after cladding with a back metal. These intermetallic compounds differ from an alloy containing only Si. More specifically, Si crystallizes as a eutectic in the form like a three-dimensionally connected coral. The crystallized Si is crushed to pieces by rolling after casting or rolling in the cladding with the back metal. Further, Si also changes its form by a subsequent heat treatment. This is a characteristic of Si and particularly in the heat treatment in which the temperature exceeds 300xc2x0 C., Si changes into a relatively rounded so that a surface tension thereof is reduced. This tendency is enhanced in a material containing a large amount of Sn, for example, an Alxe2x80x94Sn bearing alloy.
However, the aforesaid ternary intermetallic compound or multi-element intermetallic compound in the present invention does not change its crystallized form and does not change its form at a temperature for a usual heat treatment. Further, the ternary or multi-element intermetallic compound is crushed in the rolling step with plastic deformation or the cladding step during manufacture of the bearing. However, as the result of crush, the intermetallic compound takes a form with a sharp edge such as a broken piece of an edged tool. FIGS. 4 to 6 show an example of such a form. Although Si particles are rounded and broken into pieces through the steps of rolling and heat treatment, the aforesaid ternary or multi-element intermetallic compound retains an aggressive form with a sharp edge.
The ternary or multi-element intermetallic compound has a lapping effect on a counter shaft even when its amount is small. Particularly, the ternary or multi-element intermetallic compound stabilizes the relationship between a shaft with an unstable initial wear and a bearing. Thus, the ternary or multi-element intermetallic compound is effective in improving the conformability. More concretely, the ternary or multi-element intermetallic compound scrapes off protrusions on the surface of the counter shaft and an edge such as burrs around nodular graphite on the surface of the counter shaft. The ternary or multi-element intermetallic compound further prevents the Al bearing alloy from wear due to adhesion to the counter shaft, which is a disadvantage of the Al bearing alloy. Additionally, the ternary or multi-element intermetallic compound further scrapes away an adherent matter to thereby prevent seizure due to the adherent matter. Moreover, the ternary or multi-element intermetallic compound is relatively large even after the rolling step. Minutely pulverized Si particles are distributed in the Al matrix, thereby improving the strength of the Al matrix. Consequently, both improvement in the wear resistance and anti-seizure property and improvement in the fatigue strength can be achieved.
In order that the intermetallic compound having the aforesaid effect may scrape off the adherent matter on the shaft, the hard particles preferably have a maximum diameter ranging between 1 and 20 xcexcm on a bearing surface and a number of the hard particles for every 1 mm2 preferably ranges between 6 and 100, as recited in claim 3. Further, the Si particles preferably have a maximum diameter which is less than 0.5 xcexcm and a number of the Si particles for every 1 mm2 is preferably at or above 200.
The reasons for the amount limitation of each aforesaid component will be described below.
(1) Sn (3 to 40 mass %)
Sn improves surface properties such as anti-seizure property, conformability and embeddability as a bearing. When the Sn content is less than 3%, the above-mentioned effects are small. When it exceeds 40%, mechanical properties of the bearing alloy are deteriorated with the result of reduction in the bearing performance. A preferable Sn content ranges between 6 and 20%.
(2) Si (0.5 to 7 mass %)
Si dissolves in the aluminum matrix and particularly crystallizes as a single substance of silicon particle to disperse finely, so as to enhance the fatigue strength of the material and serve to improve the anti-seizure property and wear resistance. On the other hand, Si is an essential element in order to form the Alxe2x80x94Sixe2x80x94Fe intermetallic compound and improves the lapping, anti-seizure property, and wear resistance. When the Si content is less than 0.5%, Si dissolves into the Al matrix such that the above effects are small. When it exceeds 7%, its crystal is coarsened, so as to reduced the fatigue strength of the bearing alloy. A preferable Si content ranges between 2 and 6%.
(3) Fe (0.05 to 2 mass %)
Fe crystallizes mainly as the Alxe2x80x94Sixe2x80x94Fe intermetallic compound, so as to produce the above-described effects. The intermetallic compound containing Fe prevents seizure with a counter shaft and improves the wear resistance. The characteristic is effective when the Fe content ranges between 0.05 and 2%. When the Fe content is less than 0.05%, the above-mentioned effects are small. When the Fe content exceeds 2%, the compound is coarsened and the bearing alloy becomes brittle, whereupon the rolling work causes trouble. A preferable Fe content ranges between 0.07 and 1%.
(4) Mn, V, Mo, Cr, Co, Ni, and W (at least one element of these: 0.01 to 3 mass % in a total amount)
These are optional elements which constitute the multi-Element intermetallic compound in the present invention. More specifically, when a selected element xcex1 is added to Alxe2x80x94Sixe2x80x94Fe, a multi-element intermetallic compound of Alxe2x80x94Sixe2x80x94Fexe2x80x94xcex1 is produced. The selected element dissolves in the aluminum matrix as a single substance to thereby strengthen the matrix. Effects of the multi-element intermetallic compound cannot be expected when the content of each element is less than 0.01%. When the content of each element exceeds 3%, the multi-element intermetallic compound is excessively coarsened such that the physical properties of the bearing alloy are degraded and plastic workability of the bearing alloy such as rolling is degraded. A preferable content ranges between 0.2 and 2%.
(5) B, Ti and Zr (at least one of these elements: 0.01 to 2 mass % in a total amount)
These optional elements do not contribute to formation of the Alxe2x80x94Sixe2x80x94Fe intermetallic compound, dissolving in the aluminum matrix, so as to improve the fatigue strength of the bearing alloy. The aforesaid effect is small when the content is less than 0.01%. when the content exceeds 2%, the bearing alloy becomes brittle. A preferable content ranges between 0.02 and 0.5%.
(6) Cu, Mg, and Zn (at least one of these: 0.1 to 5 mass % in a total amount)
These optional elements are additional elements which improve the strength of the aluminum matrix. A solid solution treatment forces these elements to dissolve in the aluminum matrix. When the matrix dissolved these elements are cooled and aged, fine compounds can be precipitated. The effects cannot be expected when the additive amount is less than 0.1%. When the additive amount exceeds 5%, the compound becomes coarse. A preferable additive amount ranges between 0.5 and 4%.